Visual inspection of the ribbon pleats may show some heat damage (metal foils are out of shape or distorted).

Can try reshaping some of the damage with small tweezers. Or maybe just buy new ribbons and replace them. If you lift off the paper tops the ribbon is just glued on and can be removed. For new parts try locally or telephone ESS speakers in USA. They do not respond to emails. Cost maybe around US$40 each.

I opened them and...there I found.. no matter what, it was not what I sought !

After opening the "faulty" tweeter I noticed that the metal foils were in perfect order, and this puzzled me.. so I opened the other also : this was also good but not as "perfect" as the other. But there was one thing different: the glue!!
One had a transparent glue, the "faulty" had a white glue to hold it in place.

Also the color of the wires are different: one has red-blue, the other violet-blue...

In conclusion, it seems that the "faulty" AMT has a replacement ribbon. The different sensitivity can be partly caused by the different impedance, but is "normal" to have such a difference?

Cheers

Andrea / who quotes Byron (in some way..) and hopes to put an end at the disappointment